1. Field
This invention relates generally to U-tube and shell steam generators and more particularly, to such generators that buffer the heat exchange tubes from the high velocity flow of recirculation fluid and feedwater within the tube lane.
2. Description of Related Art
A pressurized water nuclear reactor steam generator typically comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a divider plate that cooperates with the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside and the feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header and through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., the side of the steam generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators. This mixture, called the downcomer flow, is conducted down the annular chamber adjacent the shell until the tube sheet located at the bottom of the annular chamber causes the water to change direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. To differentiate this steam/water mixture from the single phase downcomer flow, the fluid flow surrounding the tubes is designated as the tube bundle flow. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam and the steam vapor then exits the steam generator and is typically circulated through a turbine to generate electricity in a manner well known in the art.
Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect free by being well supported so that no breaks will occur in the U-tubes that will cause radioactive materials from the primary fluid to enter the secondary side, which would be an undesirable result. Support for the U-tubes is mainly accomplished by a plurality of transverse, spaced, tandem tube support plates that are positioned axially along the height of the tube bundle and through which the heat exchange tubes pass with their ends extending through and being affixed to the tube sheet. The holes in the support plates typically have lands that laterally support the heat exchange tubes, and lobes between the lands that permit the passage of the tube bundle flow and steam. However, tube wear has been reported at the tube support plates of steam generator units after extended periods of operation and possibly having tube and/or tube support plate fouling. The largest indications have a 28% depth. Of 79 total indications reported in one steam generator, 58, equivalent to 73% of the total number of indications, occur in rows 1-5 of the heat exchange tubes. Of these 79 total indications, 34% occur on row 1 tubes. Most of these occur at higher tube support plate elevations, where damping decreases and velocities are increased. These rows are adjacent to the tube lane region, centered between the tube hot and cold legs, and are subject to higher velocities and, thus, may experience turbulence-induced buffeting. It is well known that turbulence forces are attenuated rapidly within the first few rows of the heat exchange tubes, and the data evidences the presence of this phenomenon by the distribution of wear indications.
Accordingly, it is an object of this invention to reduce heat exchange tube wear at the tube support plates adjacent the tube lane in a tube and shell steam generator.
Furthermore, it is an object of this invention to reduce heat exchange tube wear within the vicinity of the tube support plates adjacent the tube lane without reducing the efficiency of the steam generator.
Additionally, it is an object of this invention to reduce heat exchange tube wear within the vicinity of the upper tube support plates within the first few rows of heat exchange tubes adjacent the tube lane.